The present invention relates to a multilayered air-fuel ratio sensing element which detects the emission gas concentration in an exhaust gas passage to control the air-fuel ratio of an internal combustion engine installed in an automotive vehicle or the like.
To purify the harmful emission components included in the exhaust gas of the internal combustion engine and also to improve the fuel economy of the internal combustion engine, an air-fuel ratio sensor is conventionally used to feedback control the combustion in the internal combustion the engine. A multilayered sensing element is preferably installable in such an air-fuel ratio sensor.
In general, the multilayered air-fuel ratio sensing element comprises a solid electrolytic substrate having oxygen ion conductivity, an emission gas sensing electrode provided on one surface of the solid electrolytic substrate, and a reference gas sensing electrode provided on the other surface of the solid electrolytic substrate. A surface of the emission gas sensing electrode is covered by a diffusive resistor layer with numerous pin holes allowing the emission gas to diffuse therein (refer to the unexamined Japanese patent publication 4-120454). The diffusive resistor layer functions as a diffusion rate-determining layer.
The multilayered air-fuel ratio sensing element detects an air-fuel ratio based on the current flowing between the emission gas sensing electrode and the reference gas sensing electrode when a predetermined voltage is applied between these electrodes.
The emission gas sensing electrode has catalytic activity for ionizing oxygen involved in the emission gas in response to the voltage applied between the emission gas sensing electrode and the reference gas sensing electrode. The ionized oxygen moves in the solid electrolytic substrate and reaches the reference gas sensing electrode. The ionized oxygen flow causes ion current between the two electrodes. The diffusive resistor layer suppresses the diffusion speed of the emission gas.
FIG. 14 shows the relationship between the applied voltage and the resultant current measured when the voltage is applied between the emission gas sensing electrode and the reference gas sensing electrode . As apparent from FIG. 14, the increase of the current is not always proportional to the increase of the voltage. In a specific voltage range, the current remains constant irrespective of the increase of the voltage. In other wards, a flat region appears. In general, the saturated current value in this flat region is referred to as xe2x80x9climit currentxe2x80x9d value. Hereinafter, the flat region is referred to as xe2x80x9climit currentxe2x80x9d region.
The limit current value is variable depending on the air-fuel ratio as understood from FIGS. 14 and 15. In other words, the air-fuel ratio is detectable by setting the applied voltage so as to detect the limit current.
However, the diffusive resistor layer with numerous pin holes has temperature dependency in its diffusion performance. Using such a temperature dependent diffusive resistor layer is not preferable in view of the deterioration in the measuring accuracy of the multilayered air-fuel ratio sensing element. For example, when the air-fuel ratio remains constant, there is the possibility that the limit current of the multilayered air-fuel ratio sensing element may erroneously vary due to temperature change.
One of recent requirements to be realized for the advanced automotive engines is to realize a precise engine combustion control. To this end, prompt activation of the air-fuel ratio sensor is essentially important in an engine startup condition.
The multilayered air-fuel ratio sensing element starts its sensing operation only when the temperature exceeds its activation temperature. There is a significant dead time until the temperature reaches the activation temperature in the engine startup condition. To eliminate such a dead time, the multilayered air-fuel ratio sensing element is generally equipped with a heater to warm up the sensor body as quickly as possible.
To realize prompt activation of the multilayered air-fuel ratio sensing element, it is effective to downsize the sensor body so as to reduce the overall thermal capacity. However, the downsizing is limited to a certain degree. For example, the electrode area and the diffusive resistor layer thickness cannot be reduced so much to maintain or assure the sensor performances.
Furthermore, when the diffusive resistor layer is used, the sensing current may vary in response to fluctuation of the power source voltage applied to the multilayered air-fuel ratio sensing element.
An object of the present invention is to provide a multilayered air-fuel ratio sensing element which is compact in size and is capable of detecting the air-fuel ratio accurately irrespective of the temperature change or the power source voltage change.
In order to accomplish this and other related objects, a first aspect of the present invention provides a multilayered air-fuel ratio sensing element comprising a solid electrolytic substrate having oxygen ion conductivity, a measuring gas sensing electrode provided on one surface of the solid electrolytic substrate so as to be exposed to a measuring gas, a reference gas sensing electrode provided on another surface of the solid electrolytic substrate so that the reference gas sensing electrode is exposed to a reference gas introduced into a reference gas chamber, and a porous diffusive resistor layer covering the measuring gas sensing electrode. And, a hollow space is provided between the measuring gas sensing electrode and the porous diffusive resistor layer.
According to preferable embodiments of the present invention, the hollow space has a volume of 0.2 to 3.0 mm3 per 10 mm2 surface area of the measuring gas sensing electrode. The porous diffusive resistor layer has a porous rate of 3 to 15%. At least part of a surface of the porous diffusive resistor layer is covered by a gas shielding layer. The gas shielding layer is provided at a position opposing to the measuring gas sensing electrode. The gas shielding layer is made of a gas-impervious ceramic. The gas shielding layer extends along a surface of the porous diffusive resistor layer in an opposed relationship with the measuring gas sensing electrode via the porous diffusive resistor layer, so that the measuring gas introduced into the porous diffusive resistor layer flows in parallel with the gas shielding layer and reaches the measuring gas sensing electrode via the hollow space. The hollow space has a height in a range of 20 to 150 xcexcm. The porous diffusive resistor layer is fabricated by laminating a green sheet on the solid electrolytic substrate and sintering an integrally laminated body.
Another aspect of the present invention provides a method for manufacturing a multilayered air-fuel ratio sensing element. The manufacturing method comprises the steps of preparing a plurality of green sheets to fabricate a solid electrolytic substrate, a spacer and a porous diffusive resistor layer, laminating the plurality of green sheets successively to form an integrated multilayered body with a hollow space between the solid electrolytic substrate and the porous diffusive resistor layer, and sintering the integrated multilayered body.
Preferably, the manufacturing method further comprises a step of additionally laminating a green sheet serving as a gas shielding layer on the porous diffusive resistor layer before the integrated multilayered body is sintered.